1. Field of the Invention
The present invention relates to improving scanning speed of, for example, an electrostatic capacity type position detecting device.
2. Description of the Related Art:
There are various kinds of input devices for providing position information to a computer. One such device is a touch panel capable of detecting a position on a flat detection surface where a finger or a dedicated stylus pen touches. The touch panel will output an instruction corresponding to the detected position to the computer as input information. The touch panel is widely used in PDAs (Personal Digital Assistants), ATMs (Automated Teller Machines), railway ticket-vending machines, and the like.
Touch panels can use various kinds of positional information detection technologies. For example, there is a resistance film type position detecting device that detects a position based on pressure change, an electrostatic capacity type position detecting device that detects a position based on capacitance change, and the like.
An electrostatic capacity type position detecting device will be described below. FIG. 8 is a block diagram showing an electrostatic capacity type position detecting device 801 according to the related art.
As seen in FIG. 8, the position detecting device 801 includes a driving section 802, an X-axis electrode side change-over switch 103, a Y-axis electrode side change-over switch 105, matrix electrodes 104, a receiving section 803, a position calculating section 124, and a synchronous clock generating section 807.
The driving section 802 generates an AC voltage having a frequency of 200 kHz, which is the frequency most easily absorbed by the human body. The AC voltage generated by the driving section 802 is selectively applied through the X-axis electrode side change-over switch 103 to a plurality of electrodes arranged in an X-axis direction of the matrix electrodes 104.
The matrix electrodes 104 are a sensor arranged on a flat detection surface (not shown), and are adapted to detect a position of a pointing device (not shown), such as a finger, a dedicated stylus pen, or the like. The matrix electrodes 104 are formed with a plurality of elongated conductive electrodes arranged longitudinally and latitudinally, and each intersection of the electrodes effectively forms a small-capacity capacitor. The AC voltage having a frequency of 200 kHz is applied to the small-capacity capacitors.
The Y-axis electrode side change-over switch 105 is a switch for selecting one electrode from a plurality of electrodes arranged in a Y-axis direction of the matrix electrodes 104.
The receiving section 803 is a device for converting a slight signal change, which is caused when a finger of a human body or the like approaches the matrix electrodes 104, into digital data. The receiving section 803 amplifies the signal obtained from the electrode selected by the Y-axis electrode side change-over switch 105 and performs predetermined signal processing.
The position calculating section 124, which includes a microcomputer, determines whether there is a finger on the matrix electrodes 104. The position calculating section 124 further calculates position information of the finger based on address information obtained from the synchronous clock generating section 807 and data corresponding to the slight signal change obtained from the receiving section 803.
In the following description of the matrix electrodes 104, the electrode group formed by the plurality of electrodes connected to the X-axis electrode side change-over switch 103 is referred to as X-axis electrodes 104a, and the electrode group formed by the plurality of electrodes connected to the Y-axis electrode side change-over switch 105 is referred to as Y-axis electrodes 104b. 
Next, the internal structure of the driving section 802 will be described below.
The driving section 802 includes a clock generator 109, a readout section 804, a sine wave ROM 107, a D/A converter 110, a LPF 111, and a driver 112.
The clock generator 109 is an oscillator for generating a clock that is supplied to the readout section 804. The sine wave ROM 107 has 8 bits×256 samples of pseudo sine wave data stored therein. Based on the clock supplied by the clock generator 109, the readout section 804 designates an address of the sine wave ROM 107 and reads out the data.
The data read out from the sine wave ROM 107, by the readout section 804, is D/A converted by the D/A converter 110 and smoothed by the LPF 111 so as to be converted into an analog sine wave signal. Thereafter, the voltage of the analog sine wave signal is amplified by the driver 112 and applied to the X-axis electrodes 104a as an AC voltage.
Next, the internal structure of the receiving section 803 will be described below.
The receiving section 803 includes a current-voltage converter 113, a synchronous detector 114, an A/D converter 116, a preamplifier 117, and an integrator 805.
The current-voltage converter 113, which is an inverting amplifier of an operational amplifier, is connected to the Y-axis electrode side change-over switch 105. The current-voltage converter 113 is needed because the current flowing through each of the small-capacity capacitors formed at the intersections of the X-axis electrodes 104a and Y-axis electrodes 104b is extremely small and therefore needs to be amplified and converted into a voltage.
The signal outputted from the current-voltage converter 113 is further amplified by the preamplifier 117, which is an inverting amplifier of an operational amplifier, and inputted to the synchronous detector 114.
The synchronous detector 114 is configured with an inverting amplifier 118 and a change-over switch 119. A rectangular cosine wave outputted from the readout section 804 of the driving section 802 is inputted to the change-over switch 119, and controls the change-over switch 119.
When the AC voltage is applied to the small-capacity capacitors, the phase of the current flowing through the capacitors is advanced by 90 degrees compared with the phase of the AC voltage applied to the capacitors. Thus, in order to synchronously detect the current flowing through the capacitors and into the synchronous detector 114 via the Y-axis electrode side change-over switch 105, the AC voltage inputted from the X-axis electrode side change-over switch 103 to the matrix electrodes 104 has to be shifted by 90 degrees. For this purpose, the rectangular cosine wave outputted from the readout section 804 of the driving section 802 is shifted by 90 degrees with respect to the signal having a frequency of 200 kHz outputted from the driving section 802.
The synchronous detector 114 performs a function identical to well-known diode detection of a weak signal. The signal outputted from the synchronous detector 114 is inputted to the integrator 805 configured with a resistor R120, a capacitor C122, and an operational amplifier 121. The signal outputted from the integrator 805 is inputted to the A/D converter 116. The A/D converter 116 converts the inputted analog voltage to an outputted digital value.
The position calculating section 124 serves a function of a microcomputer. Based on the data obtained from the A/D converter 116, the position calculating section 124 calculates the value of the current flowing through each of the small-capacity capacitors formed at the electrode intersections of the matrix electrodes 104. Based on the calculated value of the current, the position of the finger is detected and the result is outputted as position data.
With the position detecting device 801 according to the related art, by controlling the X-axis electrode side change-over switch 103 and the Y-axis electrode side change-over switch 105 to changeover the electrodes of the X-axis electrodes 104a and Y-axis electrodes 104b, to which the AC voltage is to be applied, a finger can be detected at each of the intersections of the matrix electrodes 104.
Diagrams (a), (b), (c), (d),(e), and (f) of FIG. 9 show waveform diagrams indicating change of signals generated by the related art position detecting device 801 and timing charts indicating operation timing of predetermined circuit portions.
Diagram (a) of FIG. 9 is a waveform diagram of the voltage of a sine wave having a frequency of 200 kHz generated by the driving section 802. The waveform of the voltage of the sine wave is detected at point P821 shown in FIG. 8.
Diagram (b) of FIG. 9 is a waveform diagram of the current generated at each of the intersections of the matrix electrodes 104. The waveform of the current is detected at point P822 shown in FIG. 8.
Note that when the AC voltage is applied to the small-capacity capacitors the phase of the waveform of diagram (b) is advanced by 90 degrees compared with the phase of the waveform of diagram (a) in FIG. 9. This is because the phase of the current flowing through the capacitors is advanced by 90 degrees compared with the phase of the AC voltage applied to the capacitors.
Diagram (c) of FIG. 9 is a waveform diagram of a signal obtained after performing a synchronous detection on the signal shown in diagram (b) of FIG. 9. The waveform shown in diagram (c) of FIG. 9 is detected at point P823 shown in FIG. 8. As shown in diagram (c) of FIG. 9, by performing the synchronous detection, the AC signal is converted into a DC pulsating current.
Diagram (d) of FIG. 9 is a waveform diagram of a signal obtained by integrating the signal of diagram (c) of FIG. 9 from time t20 to time t21 with the integrator 805. The waveform shown in diagram (d) of FIG. 9 is detected at point P824 shown in FIG. 8.
Diagram (e) of FIG. 9 shows operation timing of the A/D converter 116. The A/D converter 116 converts the analog voltage of the integrator 805, from time t21 to time t22, to a digital value.
Diagram (f) of FIG. 9 shows operation timing of a discharge switch 806. The discharge switch 806 is controlled so as to close between time t22 and time t23. By controlling the discharge switch 806 to close, the capacitor C122 is discharged, and the output voltage of the integrator 805 returns to zero.
Some of the prior art can be found in Japanese Unexamined Patent Application Publication No. 10-020992.